The present invention relates to a lustrous flaky iron oxide pigment and a method of producing the same. The pigment has a broad range of application including paints, synthetic resins, cosmetics, ink, synthetic leather and wallpaper.
Conventionally, as lustrous iron oxide pigments, a mica-like or platelike iron oxide (hereinafter abbreviated to "MIO") and a flaky red iron oxide containing aluminum in solid solution have been known. MIO exhibits a blackish-purple color and intense metallic luster (see Japanese Patent Publication (KOKOKU) No. 12435/68). However, a problem exist in that its platelike crystal structure is broken readily during dispersion into a paint or resin because of its poor mechanical strength. As a result, its color changes greatly during dispersion and it is hard to control the resulting color of a paint or a resin. A flaky red iron oxide containing aluminum in solid solution has a Al.sub.x Fe.sub.2-x O.sub.3 hematite structure and is characterized in having an intense lustrous and opacifying properties. Since the thickness of its crystal is 0.7 .mu.m or less, it exhibits a red color (see Japanese Patent Publication (KOKOKU) No. 8977/85). The red iron oxide has a high mechanical strength, accordingly its platelike crystal is resistant to breakage during dispersion into a paint or resin. As a result, any change in color is small and it is easy to control the resulting color of a paint or resin. However, because of its red color, the range of adjustable colors is restricted even if other pigments are incorporated. In addition, the flaky red iron oxide containing aluminum in solid solution is characterized by having thin crystals and there has never been synthesized a crystal having an average thickness greater than 0.8 .mu.m.
Japanese Patent Public Disclosure (KOKAI) No. 104923/80 discloses a platelike iron oxide pigment containing 0.1 to 12 weight % of at least one of the oxides of the elements belonging to Group IV, Group V and/or Group VI and/or Group B and/or Group lIB according to the periodic table. Said pigment is a mixture of an iron oxide particle and a compound of said elements and is characterized in that a hematite particle contained in said pigment has thin-plate shape of a thickness of 0.6 .mu.m or less and its color is red to red yellow. Hence, this pigment also gives rise to the problem that the range of adjustable colors are restricted.
Japanese Patent Publication (KOKOKU) No. 21976/86 discloses an improved flaky red iron oxide containing aluminum in solid solution of which surface is coated with titanium dioxide hydrate or titanium dioxide. Said pigment can be subjected to various color changes upon controlling the thickness of a titanium dioxide hydrate layer or a titanium dioxide layer on the surface of the flaky red iron oxide containing aluminum in solid solution particles. However, it suffers from a defect in that its lustrousness, an essential characteristic as a lustrous pigment, is poor in comparison with that of a red iron oxide containing aluminum in solid solution.
Japanese Patent Public Disclosure (KOKAI) No. 317559/88 discloses that colors of a pigment can be controlled variously from a copper color to a black by forming a spinel phase on the surface layer of said pigment with a reduction of the surface of a red iron oxide containing aluminum in solid solution. However, a pigment obtained by this method gives rise to problems in that its lustrous character is weak in comparison with that of a red iron oxide containing aluminum in solid solution. Furthermore, the change of color during dispersion is large, since its mechanical strength is small and its platelike crystal is readily broken during dispersion.
U.S. Pat. No. 4,826,537 discloses a pigment based on an iron oxide of the general formula Mn.sub.x Al.sub.y Fe.sub.2-(x+y) O.sub.3. The color of the pigment changes depending on the diameter of the particle. It is described that the pigment has a reddish yellow color at 10 .mu.m in diameter and which shifts toward violet with increasing diameter. However, the pigment shows violet color when its diameter becomes larger than 50 .mu.m. Such a large pigment is readily broken during dispersion and it is hard to control the resulting color of a paint or a resin.